Derivative Financial Instruments (Other Derivatives) (Details) (Not Designated As Hedging Instrument [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Derivative [Line Items]
Gain (Loss) Recognized in Income	$ (10)	[1]	$ (1)	[1]
Coal [Member] | Operating Expenses-Fuel [Member]
Derivative [Line Items]
Gain (Loss) Recognized in Income	(12)	[1]		[1]
Fuel Oils [Member] | Operating Expenses-Fuel [Member]
Derivative [Line Items]
Gain (Loss) Recognized in Income	(11)	[1]	(1)	[1]
Natural Gas (Generation) [Member] | Operating Expenses-Fuel [Member]
Derivative [Line Items]
Gain (Loss) Recognized in Income	1	[1]	2	[1]
Power [Member] | Operating Revenues-Electric [Member]
Derivative [Line Items]
Gain (Loss) Recognized in Income	12	[1]	(2)	[1]
Ameren Missouri [Member] | Natural Gas (Generation) [Member] | Operating Expenses-Fuel [Member]
Derivative [Line Items]
Gain (Loss) Recognized in Income			$ (1)

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.